Zirconia nanoparticles have been synthesized from zirconium hydroxide precipitates followed by a supercritical CO2 extraction. The microstructure evolution of these zirconia nanoparticles during the calcination at the moderate temperature has been investigated. Assisted by the analyses of TEM and XRD, small-angle X-ray scattering (SAXS) Study offers possibilities to a comprehensive and quantitative characterization of the structural evolution Oil the nanometer scales. The as-synthesized zirconia sample exhibits a mass fractal structure constructed by the Surface fractal particles. Such a structure can be preserved LIP to 300 degrees C. After calcination at 400 degrees C, considerable structural rearrangement occurs. In the interior of nanoparticles zirconia nanocrystallites emerge. It is the scattering from such zirconia nanoparticles that give rise to the broadened crossover in the vs. In q plot and the scattering peak in the ln[q(3) J(q)] vs. q(2) plot. With a further increase in the calcination temperature, the power-law region at large-q in ln J(q) vs. ln q plot expands, and the peak in ln[q(3) J(q)] vs. q(2) plot shifts towards lower q values, indicating size increases in both the nanocrystallites and nanoparticles. Besides, the mass fractal structure constructed by zirconia nanoparticles can be largely preserved during the moderate temperature calcination. (c) 2005 Elsevier Inc. All rights reserved.